I'm not sure I'd entirely agree that the BW potential of the 12PW would be wasted if crossing near the baffle-step frequency. Depends on the slope, but 1st order is likely to be popular & was mentioned above. Which is fine, but 1st order filters are a (very) slow 6dB per octave and for their potential to be fully realised, you either need drivers with a wide linear BW, or be prepared to accept extra complexity in the form of notches etc.

Many systems with very simple filters are nothing like as effective as they could be, usually due to insufficient suppression of breakup modes in the bass / midbass driver. Assuming an XO frequency of, say, 600Hz, with 1st order acoustical slopes you're only going to be 21dB down at 4.8KHz. That's not actually all that much, and if you've got an uncorrected +15dB breakup mode at, say, 4KHz in the midbass unit (which is not unheard of) you're in trouble because your low-pass transfer function and the overall frequency response have just gone to bits. Ergo, a reasonably well behaved wideband unit should be advantageous in helping to attain a linear response through the midrange without needing a bunch of extra components for supressing problems in the stop-band.

Ultimately, but not in the first octave. Of course one must consider the mechanical roll-off when doing crossovers.

Ok kind of understood and I guess I wasn't clear either. I assumed that when you add in acoustic roll off (at one end of a drivers bandwidth) then that could add potential phase issues with a 1st order i.e. better to keep the crossover in the middle, 400hz or so

I can't help but wonder how well MiniDSP would work with this driver combination at nailing down a crossover point. Or it could also be used to essentially have both a FAST system and a traditional 2-way with the tweeter crossed over beyond the vocal range.

Here's my logic on not using the A12PW in a FAST implementation. Chest thumping takes 85dB @ 80Hz at the listening chair. I have discovered this by experimentation. Any less volume, no thump. Now Mark has been very straight forward advertising the A12PW as a light weight driver. It's not going to survive as the bottom end driver at a frat party. So I will be crossing my FAST to a pair of 12" heavy duty woofers.